(1) Field of the Invention
The present invention relates to laser measurement devices, such as scanned laser radar (lidar) devices for anemometry and the like, and in particular to such laser measurement devices that are suitable for incorporation in a weatherproof housing.
(2) Description of the Art
Lidars are well known and have been used to measure wind profiles for many years. The basic principle of a lidar device is to direct a laser beam to a point or region in space and to detect the returned signal. Measurement of the Doppler shifted light back-scattered by small natural particles and droplets (aerosols) present in the atmosphere is used to provide a measure of the line of sight wind speed. In such devices the laser beam is typically scanned to enable wind velocity components to be measured at multiple points in space thereby allowing the wind vector at a remote probe volume to be calculated.
An example of an early, carbon dioxide laser based, lidar is described in Vaughan, J M et al: “Laser Doppler velocimetry applied to the measurement of local and global wind”, Wind engineering, Vol. 13, no. 1, 1989. More recently, optical fibre based lidar systems have also been developed; for example, see Karlsson et al, Applied Optics, Vol. 39, No. 21, 20 Jul. 2000 and Harris et al, Applied Optics, Vol. 40, pp 1501-1506 (2001). Optical fibre based systems offer numerous advantages over traditional gas laser based systems. For example, optical fibre based systems are relatively compact and can be fabricated using standard telecommunication components which are moderately priced and typically very reliable.
Following the recent increase in the exploitation of wind power, there is now a requirement for anemometers that are capable of making reliable wind speed measurements over prolonged periods of time (e.g. weeks or months). These routine wind speed measurements may be used to assess the suitability of proposed wind turbine sites, to measure the power extraction efficiency of existing wind turbines or even to form part of a wind turbine control system. Plans to locate wind farms off-shore has also led to a requirement to make long term, unattended, wind speed measurements away from land and WO2005/008284 (QinetiQ Ltd) describes a lidar device mounted on a buoy for deployment off-shore.
To prolong device lifetime and to minimise maintenance requirements, any commercially viable lidar device intended for outdoor use will be located within a weatherproof container. Typically, such a container will include a single optically transparent window through which radiation is both transmitted and received. For example, WO2005/008284 describes a lidar system mounted inside a buoy that has a glass window for the transmission and collection of radiation. WO2005/008284 also describes how the periodic use of a simple wiper system, or a wash-wipe system, can keep the window clear of debris. However, the activation of a wiper system requires energy from the batteries of the device and the device can only carry a finite amount of washer fluid. Overuse of a wash-wipe cleaning mechanism will thus deplete the energy and washer fluid reserves of the device thereby decreasing the time between maintenance interval. On the other hand, if the wash-wipe system is used too infrequently there may be a build up of dirt on the external window that could result in the loss, or a reduction in quality, of acquired wind speed data.